This invention relates to techniques for testing of the linearity of frequency modulated oscillators (FMO), and more particularly to the use of a low frequency modulating waveform, derived from the FMO's automatic phase control (APC) circuitry, in conjunction with the measurement of the APC control voltage when said waveform is applied to the FMO.
A frequency modulated oscillator is one whose instantaneous frequency follows an external modulating signal, called modulation. If this modulation is a symmetrical waveform and if the frequency modulated oscillator responds in a linear manner to this modulation there will be no net shift of the oscillator's average frequency while being modulated. If, however, the oscillator's response exhibits non-linearity of a certain kind, there will be a net shift in the oscillator's average frequency under modulation. This particular kind of non-linearity also happens to be one that must be carefully controlled and minimized in an FM system.
In many instances an automatic phase control (APC) circuit is used in a feedback loop around a frequency modulated oscillator (FMC) to stabilize the oscillator's frequency, usually by application of a DC correction voltage to the oscillator. If this feedback loop were broken, the oscillator would then oscillate at its free running frequency. If symmetrical modulation is applied to the oscillator and its response is non-linear, its free running frequency will shift. If the APC unit feedback loop is engaged at this time, the DC correction voltage will be adjusted to compensate for this frequency shift, thus giving a different voltage value than when the oscillator was unmodulated.
In many instances, circuitry can be incorporated within the oscillator to correct for its non-linear response to modulation. However, adjustment of this linearity correction circuitry affects the basic tuning of the oscillator, so changing its free running frequency also. Thus the DC control voltage to the oscillator will be affected by both the oscillator's non-linearity, and the detuning due to the linearity correction adjustment.
The circuitry described herein makes it possible to measure linearity and distinguish between the two above mentioned phenomena by the single expedient of alternatively switching the test modulation waveform ON and OFF while linearity is being adjusted until there is no difference in the meter reading. This is accomplished with circuitry which is intrinsic to the radio transmitter of which the FMO is a part. Previous approaches to this linearity measurement have required complex, specialized, expensive external testing equipment based upon different principles than the ones described above.